A crankshaft is commonly used in an engine and has pin and journal portions typically shaped into by machining with a crankshaft miller.
FIGS. 1A to 3B show how a pin portion of a crankshaft workpiece is machined and shaped typically, by using a crankshaft miller. A crankshaft workpiece b with a pin portion c is clamped at its ends in chucks a in the crankshaft miller in which the pin portion c is machined or milled over its peripheral surface by a cutting edge e attached along the inner peripheral surface of a ring-shaped cutter d while the cutter is in rotation.
Specifically in that method, the workpiece b carried in between the chucks a is first positioned with respect to its phase and clamped at ends thereof by the chucks a. In this stage, so that it may not disturb the workpiece b entering, the cutter d is placed on standby at a position that is concentric with the chucks a on their side as shown in FIG. 1A. The inner diameter D1 of the cutter d is larger than the outer diameter D2 of the chucks a.
In machining pin portions, starting with a first pin portion c, while the cutter d is rotated in the direction of the arrow shown in FIG. 1B and 1C, the center 02 of the cutter d located at the center 01 of the workpiece b when positioned with respect to its phase is displaced downwards as shown in FIG. 1B to permit the cutting edge e attached along the inner peripheral surface of the cutter d to machine the outer peripheral surface of the pin portion c.
Then, the center 02 of the cutter d is displaced along a circular path F centering around the center 03 of the pin portion c as shown in FIG. 1C to allow the cutting edge e attached along the inner peripheral surface of the cutter d to mill the outer peripheral surface of the pin portion c.
When machining of the first pin portion c is finished, the cutter d is displaced in the longitudinal direction of the workpiece b to index and position a pin portion c to be machined next, and a repetition of the operations mentioned above follows. With these operations repeated for all pin portions c of the workpiece b, the pin portions c are all machined.
In such use of a crankshaft miller employing a conventional inner edged cutter as described, because the cutter d is placed on standby where the chucks a are placed while the workpiece b is carried into and out of a machining area, the inner diameter D1 of the cutter d is sized to be larger than the diameter of the chucks a. Further, if the maximum swing (the largest diameter) of the workpiece b is greater than the outer diameter of the chucks a, to enable a plurality of pin portions c arranged in the longitudinal direction of the workpiece b to be machined by the cutter d on displacement in steps in that direction, the cutter d along whose inner peripheral surface the cutting edge e is attached is sized to have an inner diameter D1 which is larger than that maximum swing.
If, however, a conventional cutter d whose inner diameter D1 is greater than the outer diameter D2 of chucks a is used to machine the outer peripheral surface of a pin portion c of any diameter, disadvantageously a geometrical error may develop, resulting in the inability to achieve a machined surface of due roundness, as described below.
FIG. 2 diagrammatically shows what comes about when a pin portion c whose outer periphery has a radius r is machined with a cutter d whose inner periphery has a radius R. Assuming that the feed per cutting edge of the cutting edge e formed along the inner peripheral surface of the cutter d is f, then in the diagram of FIG. 2 EQU y=.sqroot.R.sup.2 -(f/2).sup.2, EQU y'=y-(R-r), and EQU A=.sqroot.(f/2).sup.2 -y'.sup.2.
The theoretical roundness .delta. of a machined surface of the pin portion c being represented by .delta.=r-A, it can also be seen that a surface of the pin portion c when machined with a cutter d which is larger in inner diameter becomes more polygonal as shown in FIG. 3A and has roundness reduced relative to when machined with a cutter which is smaller in inner diameter as shown in FIG. 3B. It should be noted that with the radius r of the pin portion c being 25 mm and the radius R of the cutter d being 95 mm, a feed per cutting edge f of 0.5 mm had a surface machined with a roundness of 0.00092 and a feed per cutting edge f of 1 mm had a surface machined with a roundness of 0.00368.
In comparison, when a cutter of the embodiments in accordance with the present invention as described later had an inner peripheral radius of 40 mm to machine a like dimensioned pin portion, it has been found that a feed per cutting edge f of 0.5 mm had a surface machined with a roundness of 0.00047 and a feed per cutting edge f of 1 mm had a surface machined with a roundness of 0.00188, it being seen that a much improved roundness then results as compared with the use of a conventional cutter having an inner peripheral radius r of 95 mm.
It is accordingly an object of the present invention to overcome the disadvantage found in the prior art as noted above and thus to provide a method of machining a crankshaft workpiece with a crankshaft miller which method, using a cutter which is sufficiently reduced in the diameter of its inner peripheral surface, permits the workpiece to be machined at an enhanced machining precision under a reduced influence of edge run-out and geometrical errors, as well as to provide a cutter apparatus for carrying out the method.